1. Field of the Invention
The invention relates to an apparatus for optically scanning a radiation-reflecting information plane, which apparatus comprises a diode laser supplying a scanning beam, an objective system for focusing the scanning beam to a scanning spot in the information plane and for re-imaging the scanning spot on a composite radiation-sensitive detection system, and a composite diffraction grating arranged in the radiation path between the diode laser and the objective system for deflecting the radiation reflected by the information plane towards the radiation-sensitive detection system and for splitting the diffracted beam into a plurality of sub-beams forming a corresponding plurality of radiation spots re-imaged on a corresponding plurality of detector pairs of the composite detection system. The separating strip between the two detectors of each one detector pair has an orientation such that displacements of the re-imaged radiation spots resulting from wavelength variations of the scanning beam do not have any influence on the detector signals.
2. Description of Related Art
An apparatus of this type, which is in principle suitable for reading information recorded in an optical record carrier and for optically inscribing such a record carrier, is known from U.S. Pat. No. 4,665,310. In this apparatus the composite diffraction element, in the form of a diffraction grating, fulfils two functions for which otherwise two separate elements must be used. In the first place the grating ensures that the radiation reflected by the information plane and passing through the objective system is diffracted from the path of the radiation emitted by the diode laser so that a detection system can be placed in the path of the reflected radiation. In the second place the grating splits the reflected beam into two sub-beams which are required for generating a focusing error signal, i.e. a signal comprising information about the magnitude and the direction of a deviation between the focusing plane of the objective system and the information plane. A separate detector pair is associated with each sub-beam, the difference between the output signals of the detectors associated with the same pair being a measure of the degree of focusing of the scanning beam on the information plane.
In the said record carrier the information is arranged in accordance with information tracks. If the bounding line between the two sub-gratings is parallel to the track direction, a signal comprising information about the magnitude and direction of a deviation between the centre of the scanning spot and the centre line of the information track to be scanned can be obtained by determining the sum of the output signals of each detector pair and by subtracting these sum signals from each other.
To realise the desired beam splitting, the diffraction grating of the apparatus according to U.S. Pat. No. 4,665,310 comprises two sub-gratings having the same grating period, while the grating strips of the first sub-grating extend at a first angle and the grating strips of the second sub-grating extend at a second angle, which is equal but opposite to the first angle, to the bounding line between the two sub-gratings. Since a diffraction grating diffracts an incident beam in a plane transversely of the direction of the grating lines, the beam portion incident on one of the sub-gratings will be diffracted in a different direction than the beam portion incident on the second sub-grating.
As described in U.S. Pat. No. 4,665,310 the grating design therein is based on a previously proposed composite diffraction grating. The latter composite grating comprises two sub-gratings in which the grating strips of the one sub-grating have the same direction as those of the other sub-grating, but in which the grating periods of the two sub-gratings are different. Since the angle at which an incident beam is diffracted by a grating depends on the grating period, the beam portion incident on one of the sub-gratings is diffracted at a different angle than the beam portion incident on the other sub-grating.
Satisfactory experience has been gained with scanning apparatuses comprising these gratings. However, it has been found that when using a grating a deviation in the generated focusing error signal may occur due to a variation in the wavelength of the scanning beam. It is true that this deviation can remain within the range of tolerance laid down for the focusing error signal, but it leaves only little room for possible other deviations. The last-mentioned deviations may occur, for example, due to assembly errors, movements of the optical components with respect to one another or offsets in the electronic processing circuit.
As is known, the wavelength .lambda. of the radiation beams emitted by diode lasers, which are often used in practice, may vary, for example due to temperature variations. Furthermore, the wavelengths of individual diode lasers, which have been manufactured at different instants while using the same process, may be mutually different. A wavelength variation of the scanning beam results in a change of the angles at which the sub-beams are diffracted by the sub-gratings, resulting in a change of the positions of the radiation spots on the detector pairs.
To prevent these changes in position from affecting the generated focusing error signal, it has been proposed to arrange the separating strip of each detector pair in such a way that the displacement of the radiation spots due to the wavelength variations occurs along these separating strips.
In the apparatus described in U.S. Pat. No. 4,665,310 these strips are effectively, that is to say when projected on the composite grating, transverse to the grating lines of the associated gratings. If the detector pairs are located on one side of the optical axis of the objective system, and in a plane which is perpendicular to this optical axis and coincides with or is parallel to the radiation-emitting surface of the diode laser, the separating strips will extend at equal but opposite angles (+.phi., -.phi.) to the line which connects the centre of the two detector pairs to the centre of the radiation-emitting surface of the diode laser. Such measure can also be employed in an apparatus comprising a diffraction grating whose sub-gratings have different grating periods, the directions of the grating strips of the two sub-gratings being the same.
When using a composite detector with oblique separating strips, the distance, measured along the said bounding line, between the centre of the two detector pairs and the centre of the radiation-emitting surface of the diode laser must be adjusted very accurately.